In magnetorheolgical (MR) fluids the viscosity jumps sharply under the influence of an external magnetic field – hence their utility as active dampers. MR fluids are widely modeled as Bingham plastics, with a finite yield stress that "turns on" when the magnetic field is switched on. The yield stress is commonly attributed to the formation of system-spanning chains that form due to dipole-dipole interactions. We demonstrate that this picture is too simple: computer simulations of MR fluids in bulk display a dramatic increase in viscosity – but no yield stress. We do recover a yield stress, however, in the presence of walls, indicative of a boundary effect. Since yielding is central to MR rheology, understanding this behavior is essential to validating theoretical models and developing new MR applications.

Single-walled carbon nanotubes, with their infrared emission, are promising candidates for optoelectronic purposes and in vivo and in vitro biological imaging. Their low emission yield, though, prevents their extensive use as tiny light sources.
Different approaches can be used to overcome this limitation. Among others, plasmonic systems such as specially designed nano-patterned metallic surfaces have been shown to enhance the optical response of graphene and carbon nanotubes, as demonstrated by Raman- and photoluminescence enhancement experiments.
Here I will report about nanotubes-metal hybrids based on colloidal systems. Metallic nanoparticles can be synthesized by wet-chemistry with well-controlled chemical composition, size, shape and crystalline structure, yielding LSPs with specific features. By tuning the aspect ratio and shape of the nanoparticle we realize structures with tailored electric field enhancement. This gives us the unique chance of performing a systematic investigation of the energetic overlap between metal and nanotubes. I will highlight how this can lead to optimized enhancement of the optical response of the metal-semiconductor nanohybrid systems.

A characteristic property of the 1/f-noise is its apparent unboundedness. A way out of contradiction with finiteness of the total noise power is to admit frequency-odd contributions to the noise spectral density. This option is realized by quantum fluctuations of the electromagnetic field produced by elementary charge carriers. In the present talk, the main steps of the derivation of 1/f spectrum will be described first, followed by some applications. Open problems such as deviations of the frequency exponent from unity and the correct choice of observable will be also discussed.

A defining mechanical property of amorphous materials, which are abundant in daily life and nature, is the existence of a yielding threshold. In the case of soft materials, flow occurs only when the applied stress exceeds a threshold value. Understanding the mechanisms of yielding and subsequent flow is important for a variety of rheological applications and designing newer materials. During yielding, the glassy materials often respond via formation of inhomogeneous flow patterns. These transient heterogeneities have a finite lifetime depending upon the magnitude of applied shear. We will discuss the transient dynamics that emerge during this onset of flow for various imposed shear protocols. In particular, using extensive simulations for the case of imposed shear-rates, we will demonstrate the occurrence of a transition towards the formation of a percolating cluster of mobile regions. This occurs for all imposed shear-rates and only at low shear rates, these clusters subsequently evolve into a transient (but long-lived) shear band with a diffusive growth of its width, before steady flow is attained.

In this talk I will consider single particle Standard Model extensions in order to solve the discrepancy between the measured and the predicted value of the anomalous magnetic moment on the muon. After classifying all the scalar, fermionic and vectorial extensions, I will discuss in each case whether the new physics can solve the anomaly or not. I will give particular attention to vectorial extensions, which correspond to non-renormalizable models, and show how calculabitily can be recovered if the massive vectors are the gauge bosons of a broken enlarged gauge symmetry.

Depuis l’observation de l’émission de lumière des nanotubes de carbone (NTC) en 2002 par O’Connell et al., les propriétés optiques de ces nano-objets unidimensionnels ont fait l’objet d’un nombre considérable d’études de recherche reportées dans la littérature. Afin d’optimiser les fonctions optiques pour les réseaux de télécommunications optiques, et d’ouvrir la voie des composants optiques de nouvelle génération à base de NTC, nous étudions les propriétés optiques linéaires et non-linéaires des NTC, comparativement aux hétérostructures à semiconducteurs III-V conventionnelles (puits, fils et boîtes quantiques).
Je présenterai au cours de ce séminaire nos résultats sur les propriétés d’absorption non-linéaire des NTC pour les applications en tant qu’absorbants saturables ultra-rapides, puis nos études sur leur intégration dans un guide d’onde optique qui ont abouti à la 1ère observation de photoluminescence guidée des NTC à 1,55um. Ces études sur les composants optiques passifs à base de NTC, réalisés technologiquement au laboratoire FOTON, sont une étape nécessaire vers la fabrication et la démonstration de composants actifs performants tels que des sources de lumière à base de NTC (type DEL et lasers). Depuis la 1ère observation en 2010 du gain optique qui constituent toujours un challenge international.

Some remarkably universal empirical formulas are used to characterize the slow dynamics of disordered materials and imperfect crystals. Stretched-exponential relaxation is used for time-dependent response, non-classical critical scaling is used for temperature-dependent behavior, and 1/f noise is used for frequency-dependent fluctuations. I will describe a common physical foundation for all of these formulas. The behavior may be attributed to strict adherence to the laws of thermodynamics: energy is conserved using Hill’s subdivision potential for nonlinear terms, entropy is maximized using a local thermal bath, and similar states are treated using the statistics of indistinguishable particles. Alternatively, the mechanism may be interpreted using information theory for reversible fluctuations. I will emphasize how specific models based on these principles yield the empirical formulas, plus deviations from the formulas that often match the measured behavior. 

The insertion of defect operators of various types can change the Hilbert space of a quantum field theory. Surface defects in 4d N=2 theories introduce a class of ”2d-4d” BPS states which the original 4d theory does not possess. For theories of class S spectral networks count 2d-4d BPS states, and through the 2d-4d wall-crossing phenomenon the 4d BPS spectrum can be obtained. In this talk I will give a brief introduction to spectral networks, with an emphasis on their physical interpretation. Some of their applications and extensions will then be illustrated.

Les expériences (e,2e) et (e,3e) constituent un outil privilégié pour étudier la dynamique de la simple et la double ionisation de petits systèmes par impact électronique, et plus généralement pour contribuer à la compréhension du problème de l’interaction à N-corps.
Dans ce travail, nous avons réalisé de telles expériences dans un domaine cinématique resté quasi-inexploré, où le transfert de moment à l’ion résiduel est important, de sorte que l’ion joue un rôle majeur dans le processus d’interaction projectile - cible. Les résultats expérimentaux sont comparés à ceux des modèles théoriques les plus sophistiqués.
Je vous montre les sections efficaces triplement différentielles (SETD) de simple ionisation de He et H2, ce qui a permis de mettre en évidence la présence pour la molécule H2 d’effets d’interférences quantiques de type fentes d’Young.
Nous discutons ensuite des mesures de SETD de Ne et N2, ionisés sur une orbitale externe ou interne. Nos résultats ont permis de montrer l’importance des interactions post-collisionnelles et du rôle joué par le noyau.
La compétition entre différents processus d’ionisation de l’argon sera aussi discutée : d’une part, simple ionisation (e,2e) en couche interne 2p, et d’autre part, double ionisation (e,3e) directe (3p-2) ou indirecte via le processus Auger impliquant la couche 2p. Dans les conditions cinématiques choisies, ces processus peuvent entrer en compétition ou interférer entre eux. L’accent est mis sur leur contribution respective, en particulier pour l'effet Auger. Diverses structures observées dans la distribution angulaire des sections efficaces (e,3e) sont attribuées à différents mécanismes d’ionisation.